In a mixed crowd, typical consumption is an average of 1/4 to 1/2 pound per person. Split the difference and call it 1/3 pound each. Your net yield from a pork butt will be 50-60% of the uncooked weight. You would need about 25 pounds uncooked, minimum. I'd cook four average 7-8 pounders. If you're buying them in cryovac, that's the fewest you could purchase and be sure you had enough. If you have a vacuum sealer, nothing will go to waste, or you can send some home with guests.